69 lines
2.3 KiB
Text
69 lines
2.3 KiB
Text
VideoCore IV support in the ACK
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===============================
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This is a fairly crude port of the ACK to produce VideoCore IV machine
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code, suitable for use on the Raspberry Pi. It produces terrible but
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working code. The resulting binaries can be used either bare metal or
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loaded as a GPU kernel and executed using a modified mailbox.c (see below).
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Currently floating point support is present but incomplete; and as the
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VideoCore IV does not have double-precision float support, the C compiler
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treats doubles as single precision.
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As much of the standard C library as is relevant works; if
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you're running in bare-metal mode, you can hook stdin/stdout up to the
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mini UART. (Obviously, in kernel mode you can't.)
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Important note! The malloc heap expects your program to be loaded into a
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chunk of memory that's 128kB large. You must make sure that this is the case,
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or Bad Stuff will happen.
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Output binaries are fully PIC and can be loaded anywhere (this is one of the
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things that makes the code so terrible). You must use the pi_user_to_phys()
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and pi_phys_to_user() to translate pointers from physical to user and vice
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versa. If you don't, Bad Stuff will happen.
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Bare metal mode
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---------------
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To run a binary bare metal, compile it:
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ack -mrpi -O program.c -o bootcode.bin
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...and copy the bootcode.bin file to the root of an SD card. Boot the Pi.
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Your program will run.
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To use the UART, #include <pi.h> and call pi_init_uart() at the top of your
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program. This will set it up and connect it to stdin/stdout. It's 115200 8n1.
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Kernel mode
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-----------
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This will require some hacking at your end.
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Go here, and follow the instructions.
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https://github.com/hermanhermitage/videocoreiv/wiki/VideoCore-IV-Kernels-under-Linux
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Now compile your program:
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ack -mrpi -O program.c -o alpha.bin
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MAKE SURE YOU AREN'T USING ANY MEMORY ALLOCATION. Copy the alpha.bin onto
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the Pi, and run it with mailbox.c.
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To get data in and out, #include <pi.h> and look at the pi_kernel_parameters
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variable. It's a structure that is initialised with the data that's passed in
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from mailbox.c (currently four pointers and two integers).
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If you want to use malloc() and friends, you'll need to hack mailbox.c so
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that the buffer containing the code is at least 128kB, or you're likely to
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corrupt the VideoCore's workspace and crash it.
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David Given <dg@cowlark.com>
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2013-06-06
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